For both human (HIV) and simian (SIV) immunodeficiency viruses, vaccines have failed to generate antibodies that can cross-neutralize genetically divergent isolates. Moreover, with the exception of an attenuated SIV, these preparations have generally failed to produce protective immune responses in animal models. One possible explanation for this failure has been that synthetic, recombinant or killed virus preparations utilized to date have not preserved the structural integrity of viral envelope glycoproteins present on virions and infected cells. Interestingly, recent studies of HIV-1 have indicated that when soluble oligomeric rather than monomeric envelope glycoproteins are used as immunogens, neutralizing antibodies can be produced whose epitopes are defined by the quaternary structure of this multimeric complex. This finding has indicated that there are potentially important components of the humoral immune response to HIV that become evident only when envelope glycoproteins are presented to the immune system in their native multimeric conformation, and which have not been examined thus far. As will be described, these observations have recently been extended to an in vitro SIV model in which an SIV was genetically altered to express increased levels of conformationally-intact envelope glycoproteins on the surface of infected cells. When these cell- associated envelope molecules were used as an immunogen in mice, a novel panel of monoclonal antibodies was produced that exhibited potent and cross-neutralizing activity for several genetically divergent isolates of SIV. These preliminary findings have indicated that similar to HIV-1, SIV envelope glycoproteins can also elicit a strong and broadly-reactive neutralizing immune response when their native multimeric conformation is maintained. Clearly, a thorough evaluation of this humoral response is needed for SIV where its relevance to protective immunity can be determined. This proposal will rigorously assess the humoral immune response to novel preparations of soluble and cell-associated oligomeric envelope glycoproteins of SIV in both small animal and non-human primate models. Specifically, plans are described, 1) to produce and characterize soluble and membrane-associated oligomeric SIV envelope molecules as immunogens; 2) to produce monoclonal and polyclonal antibodies to these immunogens in order to characterize their antigenic structure and to determine their ability to elicit neutralizing antibodies to genetically divergent isolates; and 3) to analyze the humoral responses in non-human primates and determine if protection can be achieved from a viral challenge. These studies will add considerably to an understanding of the contribution of envelope glycoprotein quaternary structure to its immunogenicity and determine its potential use in developing an effective vaccine for HIV.